1. Field of the Invention
This disclosure relates generally to a closed drift Hall type ion accelerator in a vacuum space, and more particularly to a closed drift Hall Current accelerator operating radially across an axial magnetic field gap between a pair of split solenoid windings.
2. Background of the Invention
Hall accelerators generally operate by accelerating ions along the axis of some form of a solenoid type magnetic field. Most commonly, as in single stage Hall thrusters, the gas is sourced from within the solenoid structure, and the ions are accelerated axially through an open fringe field. In the less common two stage Hall accelerator, the gas is sourced from generally outside the solenoid field but the acceleration is still axial, across the radial aspect of the solenoid fringe field. In such devices the axial field component is minimized as well as its influence on the ion trajectories. Hall Effect accelerators are designed to capture electrons in Hall Effect drift orbits and therefore need to have acceleration channels with a width greater than the electron orbit gyro-radius. The ion trajectories are also bent in the Hall Effect magnetic field, however they are only allowed to bend a very small amount so that their trajectories remain essentially axial. Hall Effect accelerators have not been used to accelerate ions into a solenoid field. The prior art involves accelerating ions axially, out of or through the magnetic field.
In an attempt to transport ions into a solenoid, ions have been injected axially as well as radially. Axial injection involves cross field transport across the fringe field. Radial injection involves transport across the solenoid return field. Plasma beam transport across magnetic field flux proceeds by one of three effects. At low densities, ions transport across the magnetic field according to classical single particle dynamics. At medium densities, plasma beams become electrically polarized by the magnetic field. The polarization electric field tends to keep the beams together, counteracting the tendency of the beam to bend under the influence of the magnetic field, thereby allowing the beam to propagate relatively un-deviated across the field. At high densities, the beam plasma excludes the magnetic field from the interior of the beam and the beam passes without deviation across the field. These effects are the result of the fact that the ions and electrons are separated upon encountering a magnetic field. Hall Effect accelerators operate without ion electron separation.
The following references illustrate some of the prior art with regard to Hall Current accelerators and cross field charged particle transport. Raitses et. al. in U.S. Pat. No. 6,448,721 reveals an example of the progression of Hall accelerators from the common annular design towards a reduction of the inner electrode with their cylindrical geometry Hall accelerator. The design references a single stage Hall accelerator. Fisch et. al in U.S. Pat. No. 6,777,862 discloses a segmented electrode Hall thruster with reduced plume which addresses the importance of reducing plume divergence. Mahoney et. al. in U.S. Pat. Nos. 5,973,447 and 6,086,962 discloses a gridless Hall effect ion source for the vacuum processing of materials. Kornfeld et. al. in U.S. Pat. Nos. 6,523,338 and 7,075,095 discloses plasma accelerators using multi-acceleration stages. Cann in U.S. Pat. No. 3,309,873 discloses a plasma accelerator utilizing a Laval type nozzle, and Cann in U.S. Pat. No. 3,388,291 discloses an annular array of multiple collimating anode gas sources. W. H. Bennett in U.S. Pat. No. 3,120,475 claims a means of injecting ions radially into a magnetic field, at a position that is off the center and off the axis of the magnetic field chamber. Maglich in U.S. Pat. No. 4,788,024 discloses radial injection of a beam of charged particles into a magnetic field inside a vacuum zone, thereby producing ions that generally have zero canonical angular momentum. Kapetanakos in U.S. Pat. No. 4,293,794 reveals a method of pulsed, full cusp, cross field transport of ions into a solenoid field, with a half cusp beam exit.
Numerous articles have been published detailing the electro-dynamic processes at work when plasmas are transported across generally transverse magnetic fields. To name two: “Propagation of intense plasma and ion beams across B-field in vacuum and magnetized plasma” by Vitaly Bystritskii et. al. published in Laser and Particle Beams (2005), 23, 117-129. Another is “Propagation of neutralized plasma beams” by N. Rostoker et. al. published in Phys. Fluids B 2 (6), June 1990.